In order to increase the starting torque of a motor and to decrease the starting current, a wound-rotor or a cage with a special slot, such as a deep slot or double cage, etc., is used for asynchronous motors. A damping cage is used for asynchronous starting at low voltage for a synchronous motor. At the present, there are many publications about the solution or improvement of the starting torque and starting current, for example, a Chinese publication "Technical information on medium and small electric machines" 1979, No. 1, P. 16-20 and 1980, No. 4, P. 27-30; USSR Pat. No. 756550, etc. The starting method of a wound-rotor asynchronous motor without slip rings and extra starting resistors, which is proposed in "Technical information on medium and small electric machines" 1979, No. 1 and 1980, No. 4, is to change the connection of rotor windings when the motor starts, by opening and closing three contact points in the rotor winding. Although the motor, which is provided in the literature, has eliminated the slip rings and the brushes, it is necessary to have a means for opening and closing the contact points when the rotor rotates which complicates the construction of the rotor and decreases the reliability of the motor. The motor, by means of changing the connection of the rotor winding, increases the resistance and the reactance of the rotor referred to as the stator side, and decreases the starting current and increases the starting torque. However, the starting torque does not increase much due to the increase of the reactance and the decrease of the maximum torque as well, so that it cannot be used where a large starting torque is needed.
USSR Pat. No. 756550 proposes a three-function winding for starting, excitation and damping of a four-pole synchronous motor. It uses the fundamental wave to start the motor with the same pole numbers for starting and running. The three-phase windings on the rotor are: from phase 1 to phase 2 will be displaced 60.degree., while from phase 2 to phase 3 will be displaced 60.degree., and from phase 3 to phase 1 displaced 240.degree. electric degrees. It is not a symmetric three-phase system. Such a motor will provide a large starting torque only when the two terminals for DC excitation are short-circuited. It is not easy to reach synchronous speed for heavy loads. The heavier the load is, the more difficult it is to pull into step (or synchronization) after the DC is applied. In addition, it must start with a lower voltage, in order to decrease the starting current, so as to decrease the starting torque. The three-function winding of the motor cannot be connected with starting resistors in series during the starting period, otherwise it will cause the three-phase current to be more asymmetric. Since the starting current and the direct-current excitation of the three-function winding flow in the same path and interfere with each other, they will not exist at the same time. In brief, the motors and other starting methods which are widely used now have many problems in the manufacture and the running of the motor, such as complicated construction, low reliability of running, high cost, having a large impulse of current occur during the starting process, interference to power network, shortening of the service life of the motor, insufficient starting torque, difficulty in pulling into step, etc. The efficiency of a cage asynchronous motor is not very high because of the large copper loss of the rotor.
The object of the present invention is to provide a starting method to overcome these disadvantages and to offer a general way to design such windings. According to this design, it is easy to produce a series of motors with simple construction and good starting performance.
The starting method of an AC motor will directly affect the performance of the motor, the construction of the motor, and the extra apparatus which is needed during the starting process. Finally, it will affect the running performance of the motor and the reliability of the whole procedure from starting to running. The current starting methods utilized or proposed are not able to offer a good solution to all the problems listed above.